Field of Invention
The present invention belongs to a magnetic resonance imaging field, and relates to an ultrahigh resolution magnetic resonance imaging method and apparatus.
Description of Related Arts
A magnetic resonance imaging (MRI) apparatus is an imaging diagnosis apparatus that excites nuclear spins of a test body placed in a superposition of a static magnetic field and a gradient magnetic field by a radio frequency (RF) signal corresponding to their Larmor frequency, and to reconstruct an image according to the magnetic resonance (MR) signal generated with the excitation and space distribution information of the gradient magnetic field.
The magnetic resonance imaging technology has become a necessary diagnostic tool for modern medical diagnosis, since it may carry out non-destructive imaging on a human body. It utilizes the theory of nuclear magnetic resonance, and adopts a method that adds the gradient magnetic field to obtain the space position information of the nuclear spin according to the corresponding relation between the nuclear magnetic resonance frequency and a magnitude of the magnetic field. Since the nuclear spins inherently have relative lower polarizability, and on the assumption that the nuclear spin density is constant, only the sample with relative larger volume is able to obtain sufficient spin polarization to generate a reasonable signal. As a result, the magnetic resonance imaging resolution is limited and remains at the order of millimeter. Therefore, it requires for improving two basic factors to improve the magnetic resonance imaging resolution to detect the microscopic world, wherein, one is to improve the magnetic field gradient, and another having a higher difficulty level is to improve the detecting sensibility of the spins.
Currently, a magnetic resonance force microscopy (MRFM), which utilizes a very sensitive cantilever to detect force interaction between spins and a nanomagnet, may achieve a magnetic resonance imaging with a resolution of 4˜6 nanometers. Since such detection technology is to transform the spin physical quantity into a weak force measurement through its interaction with the nanomagnet, and thus is highly susceptible to disturbance of other environmental factors, such as an electric field force, thermal fluctuation, ambient mechanic vibration, etc. Meanwhile, it has been proposed a conception of using a nitrogen-vacancy center (NV center) in a diamond as a ultra-sensitive spin detector to achieve a nano-magnetic resonance imaging in recent years, while there are still many technical difficulties to be solved, such as, the NV center in the diamond fails to work in a relative larger magnetic field environment, with the result that it is impossible to highly polarize the nuclear spins of the test body to generate relative larger signals. Besides, the NV center is generally located inside the diamond, and is thus relative far away from the surface, while its coupling effect with the spins quickly attenuates with distance, thus its imaging scope is also relative small.
A Chinese patent, with a publication number of CN1643403A, has disclosed a NMR (nuclear magnetic resonance) and a MRI (magnetic resonance imaging) detected under a extremely low field SQUID (superconducting quantum interference device), which utilizes the SQUID of high sensitivity to measure the spin signal under a extremely low magnetic field. The working principle thereof is that: transforming a spin signal of an atom into a current signal by a pickup coil, transforming the current signal into a magnetic signal, and utilizing the SQUID to measure the signal, wherein, the SQUID actually plays a role of an amplifier of the current signal. In such method, the sample is separated with the SQUID, that is, the sample is at room temperature, and the SQUID is isolated below the superconducting critical temperature; since the conventional SQUID is susceptible to the effect of environmental magnetic field, generally in practice, the SQUID is packaged in magnetically shielding environment, and the magnetic signal of the sample is transferred to the SQUID through the pickup coil, which is also a commonly used measurement arrangement of the conventional SQUID. The main advantage of the MRI imaging by ultra-low field SQUID in the patent is to cancel the dependence of the MRI on a huge static magnetic field B0, thereby it is achievable to utilize the earth magnetic field as a MRI imaging condition. Unfortunately, although the measurement arrangement of the conventional SQUID has much higher sensitivity than that of a detector used in a commercial MRI apparatus, due to the limitation of the pickup coil, most magnetic signals of the sample are lost during the transfer process. Therefore, when measuring a tiny sample or microscopic spins, the conventional SQUID detector may not greatly exhibit its full potential.
The nano-scale superconducting quantum interference device (nanoSQUID) is a novel device developed based on the conventional SQUID, which utilizes a nano junction to replace a conventional tunnel junction, so that the area of the superconducting ring can be greatly decreased, and coupling degree between the device and the tiny sample can be greatly enhanced. As comparing to the magnetic resonance force microscopy and the NV center in the diamond, the nano-scale superconducting quantum interference device (nanoSQUID) features comparative or better spin sensitivity. Moreover, the device performs direct and close range detections by a magnetic flux coupling, which is not influenced by vibrations and electric field; besides, the device may be normally operated under a strong magnetic field, which means the thermal equilibrium polarization of the nuclear spins can be increased by an external static magnetic field. Therefore, the magnetic resonance imaging of nano-scale resolution is achievable by utilizing the nanoSQUID as a detector and by cooperating with a large gradient magnetic field.